1. Field of the Invention
The invention relates in general to an AC/DC converter capable of actively restraining an inrush current, and more particularly an AC/DC converter that can effectively restrain an inrush current in any working environment and on any occasion, so as to ensure the stability of a power supply.
2. Description of the Related Art
With reference to FIG. 12, an AC/DC converter for a power supply includes a bridge rectifier 80, an inductor 81, a power transistor 82, a diode and a capacitor 83. The bridge rectifier 80 is used for converting AC power to DC power. The inductor 81 is connected to the bridge rectifier 80. A power factor and pulse width modulation controller 84 controls the power transistor 82. An auxiliary charge circuit including a resistor R1 and a diode D1 assists the capacitor 83 to charge. A main function of the auxiliary charge circuit is to setup an appropriate voltage of the capacitor 83 before a power factor correction circuit starts, so that an auxiliary power source can operate accurately.
An inrush current occurs when the power supply starts because the capacitor 83 has no charge. A conventional technique to prevent components from being damaged by the inrush current is to use a thermal resistor 85 connected between an output terminal of the bridge rectifier 80 and the inductor 81. Two terminals of the thermal resistor 85 are connected in parallel to an electrical switch 86 to determine an occasion to interrupt the circuit. The auxiliary power source inside the power supply controls the electrical switch 86 to switch on or off.
With reference to FIG. 13, a standard start-up timing diagram of a power supply is illustrated. An AC power source VAC starts to be input to the power supply on startup. The capacitor 83 is charged through the thermal resistor 85, the inductor 81, the resistor R1, and the diodes D1, D2. A voltage VBulk gradually rises until it reaches a certain voltage value. The curve of the voltage VBulk in the diagram shows that the inrush current has been effectively restrained. Next the auxiliary power source Vstandby starts and gradually rises until it reaches a set voltage for a period of time. Then the power supply starts to supply an output voltage Vout.
The electrical switch 86 that controls whether or not the thermal resistor 85 should interfere in the circuit is controlled by the auxiliary power source and related circuits. The electrical switch 86 is opened before the power startup. That is to say, the thermal resistor 85 connected between the bridge rectifier 80 and the inductor 81 can restrain the inrush current after the power startup despite the capacitor having no charge. Because the inrush current only occurs at an instance of startup, after the power supply operates normally the thermal resistor 85 must be separated from the circuit, so that the thermal resistor 85 will not reduce efficiency. Therefore the electrical switch 86 closes when the auxiliary power source of the power supply is setup. The DC power then immediately goes through the electrical switch 86 to the inductor 81 without passing through the thermal resistor 85.
The aforesaid technique to control the inrush current is practicable. However, the aforesaid technique is limited and has drawbacks in practice. To be specific, the aforesaid technique is only suitable for cold startup, which refers to startups from a total power off state. During warm startups, especially occasions of warm startup reboots, the aforesaid technique may lose efficacy. The root cause is because the electrical switch 86 is controlled by the auxiliary power source. As shown in FIG. 13, even though the AC power source VAC interrupts, the auxiliary power source Vstandby still continues for a period of time of t4-t6. In other words, during the AC power source VAC interruption or warm startup, the electrical switch 86 still remains closed since the auxiliary power source Vstandby continues to supply power. This indicates that the thermal resistor 85 is isolated from the circuit. Hence the thermal resistor 85 is unable to restrain the inrush current in the aforesaid occasions.
FIG. 14 illustrates a method and circuit for controlling inrush current disclosed in an U.S. Patent Application No. 2003/0035311. A circuit in this patent includes a bridge rectifier 94, a boost converter 120, a capacitor 33 connected between positive and negative power terminals and a resistor 92 arranged from AC power terminals AC L and AC N to power output terminals BULK+, BULK− in sequence. The resistor 92 is connected in parallel with a switch transistor 119. A gate of the switch transistor 119 is connected to a control circuit 110 for controlling inrush current.
The control circuit 110 for controlling the inrush current includes comparators 620, 640, 660, 102, 114 and a transistor 118. Input terminals of the two comparators 620, 640 located at a front end are connected to an output terminal of the bridge rectifier 94 through voltage divider resistors 630, 650, 680, 690 to detect an output current variation. When an output current the bridge rectifier 94 goes through the comparators 620, 640, 660, 102, 114 to be compared until the voltage exceeds a predetermined threshold, the inrush current occurs. At this moment, the comparator 114 outputs a low voltage to make the switch transistor 119 open, and the resistor 92 becomes connected in series with the capacitor 33, so as to restrain the inrush current.
However, although the aforesaid technique can restrain the inrush current during startup; there is still a drawback to this technique. The terminal voltage BULK+ of the capacitor 33 is supposed-to be rising gradually. When the resistor 92 interferes with the circuit, the current goes through the resistor 92, and thereby the voltage BULK+ rises abruptly. Hence the terminal voltage BULK of the capacitor 33 encounters excess voltage and is likely to impact a voltage tolerance of the capacitor 33.